There is a conventional nitride semiconductor device in which a surface of an n-type GaN contact layer has been subjected to oxygen plasma treatment to form an oxygen-doped layer at the surface thereof prior to formation of an ohmic electrode on the n-type GaN contact layer for the purpose of reducing the contact resistance between the ohmic electrode and the contact layer (see Japanese Patent No. 2967743 (PTL 1)).
However, as a result of experiments, done by the present inventors, in which an ohmic electrode was formed on a GaN layer after subjecting the GaN layer to the oxygen plasma treatment, contact resistances obtained of the ohmic electrode were found high. The inventors were not able to obtain a sufficiently reduced contact resistance.